I have a problem with a date formula in a spreadsheet that I would very much appreciate help with. It seems that the formula doesn't work past the new year for some reason. My formula is set as follows:

Cell Z3 has a date on it, cell AA3 should populate with a future date 4 weeks from now. My formula is =IF(Z3="","",Z3+7*4). This works as long as the future date doesn't go past the new year. If the date is for 2018, it gives #VALUE as a result.

Thank you for your quick reply. Yes unfortunately, the problem is still there. I've checked that the formatting is definitely as date. Could it be because it's Excel online? I'm totally baffled by this, and worry I won't be able to give an answer to my client.

"...Could it be because it's Excel online? ..."
I do not think it would make any sense if it happened.
It is a basic operation.
And you're using EXCEL, with the only difference being the online version.

I've downloaded the spreadsheet as it's stored in O365 as a shared document. The formula on the download works fine and gives the dates for 2018. I've created the formulas on the shared document and have access to update it on the browser (allows to make quick changes) but not on the "edit in Excel" option which would give full functionality of the spreadsheet. They are migrating from google docs and that stops me from accessing the "edit in excel" as I'm an outside contractor and work from home.